In disc tumbler cylinder locks, tumbler discs are used to resolve a key-specific code and open the lock. Inserting a key into a disc tumbler cylinder lock does not yet cause the code to be resolved, rather it is the turning of the key that causes turning of the tumbler discs according to the grooves of the key and thus resolution of the key code.
It has been observed that if the key is, for some reason, not fully inserted into a disc tumbler cylinder lock, then turns of the key can, in this case, cause a very slight turning of the tumbler discs away from their common standard position. The standard position means that the key can be inserted into the lock. If some of the tumbler discs have turned a tiny bit from the standard position, then inserting the key into the lock does not go smoothly, it will instead be experienced as difficult or impossible. The key will have to be turned several times from side to side in order to get the key fully into the cylinder lock. To prevent this, disc tumbler cylinder locks are often provided with a rotation limiter.
A rotation limiter is a mechanism, which prevents a key from turning in a disc tumbler cylinder lock, if the key is not fully inserted into the cylinder. The rotation limiter allows a key in a cylinder lock to be turned, when the key is fully inserted into the cylinder. Patent publication FI 108308 presents a known rotation limiter of a disc tumbler cylinder lock. The rotation limiter comprises a frame and a casing. The casing is connected to the inner cylinder of the disc tumbler cylinder lock. In the centre of the frame is a key profile opening, through which the key can be inserted into the cylinder lock. On both sides of the key profile opening is a limiting mechanism in the same line. The limiting mechanism is made up of a spring and balls on both sides of the spring. If the key is not fully inserted into the cylinder lock, the surface of the shaft of the key keeps the balls of the limiting mechanism so close to each other that, if an attempt is made to turn the key, the ball on the casing side is unable to move away from the hollow of the casing. Because the casing is connected to the inner cylinder, which is not able to turn until the key code is fully resolved, a key that is incompletely inserted into the cylinder lock is not able to turn.
The key has counter surfaces for the balls of the limiting mechanism. The counter surfaces are recesses in the surface of the key. When the key is in the correct place in the key cylinder, the ball of the limiting mechanism on the key side is in the recess of the key, and as the key is turned, the ball on the casing side is able to move away from the hollow of the casing. Thus, the key can be turned in order to move the tumbler discs into the correct position for opening the lock. In this case, the inner cylinder is able to turn as turning of the key is continued.
The rotation limiter makes the disc tumbler cylinder lock more functionally reliable. However, it is able to rotate in the cylinder lock, when a key is not inserted into the lock. This rotation facilitates the possible picking of the lock.